The present invention relates to an optical operating element, more particularly a pushbutton or switch, comprising a light transmitter, an optical receiver, a prism suitable for guiding light, and having a cover that is impermeable to radiation and has a least one exit aperture.
Contact switches or pushbuttons that trigger a switching operation in a device when touched by the user are used widely in many electronic devices, such as mobile phones, remote control devices, domestic appliances and lifts, as well as in the automotive industry. Both capacitive and optical switches are known for this function. Capacitive contact switches have a sensor surface that as part of a capacitive sensor element generates a capacitance in conjunction with the user, which changes correspondingly when touched. The sensor surface is usually coupled via a measuring electrode with a circuit arrangement that feeds a corresponding output signal to an evaluation circuit. From this, a determination may be made as to whether the contact sensor is being touched or not.
However, capacitive measuring methods have only small ranges, and are essentially limited to direct contact. If the range of this sensor is increased, the sensors usually respond incorrectly or at least unbidden to moisture, temperature changes or stray electromagnetic radiation. Moreover, these sensors do not work when surrounded by conductive surfaces such as metal or metalized surfaces.
Besides capacitive operating elements, optical elements are also known, and in these the reflection of light in the visible or invisible range, particularly the infrared range is used. Thus, it is possible to detect not only contact, but also the approach of an object, a finger for example, by detecting and evaluating changes in the quantity of reflected light. By analyzing the reflected light, a change in the reflected light is detected, and this then triggers switching or similar operation. Although the operating elements based on optical measuring methods have a greater range, they also often have the disadvantage of being susceptible to interference by extraneous light. They also require a certain installation space behind the user interface for locating the sensors (photoreceivers). They always require a user interface that is translucent to the extent that sufficient light is radiated and a sufficiently large fraction of reflected light can be received and forwarded to the optical sensor.
Optical sensors preferably use LEDs as light sources and light sensors. Photodiodes or photoelements are used as receivers. The sensitivity of photodiodes to modulated light is dependent on absolute brightness, and this dependence is called the ambient light effect. Accordingly, the received signal of a reflection light barrier depends not only on the degree of reflection that is to be measured, but also on the ambient light. It is not possible to evaluate the received signal reliably if the ambient light conditions are too variable. Document JP 2006-164672 A describes an optical sensor that is arranged behind a mirror. The mirror has a certain number of apertures in the silver film thereof, behind which an optical prism is arranged to guide the light and illuminate the sensor. The optical sensor is disposed in such manner that light is emitted directly through one of the apertures in the silver film, and the light reflected by an approaching object is received by a receiver positioned directly behind the aperture. The emitter and the receiver are arranged side by side, which results in crosstalk and the sensitivity of the sensor is impaired to such a degree that an object approaching the sensor cannot be detected reliably.
A measuring method is known from EP 0 706 648 B1 in which the transmission function of the photodiode is eliminated. At least two light sources alternate in transmitting light to at least one receiver. The light sources are controlled in such manner that from the viewpoint of the receiver they are constantly equally bright. Consequently, the signal received by the photodiode is of equal magnitude for both signals. If the two light sources transmit oppositely phased rectangular light signals, adjustment of the measurement path yields a received signal of zero. The consequence of this is that the receiver amplification may be almost infinite since only the receiver signal filtered by a band-pass is evaluated. If one of the light sources is too strong, the subsequent circuit detects this fact and readjusts the intensity of the light source accordingly until the differential signal of the two light sources at the receiver is equal to zero again.
Document EP 1 671 160 B1 discloses a method for determining and evaluating a differential optical signal. This method is based on EP 0 706 648 B1 and also comprises a compensation light source for introducing the adjustment value back into to the adjustment path.
The optical sensors based on reflection always require the use of a sensor surface through which a sufficient fraction of the light is able to escape. In the related art, there is a substantial need for optical operating elements in which the sensor surface that serves as the functional surface or pushbutton surface only has a very low light transmittance. This may be because, for example, materials with low transmittance or materials that have large ranges of dispersion are used, with the result that a light beam that is reflected outside of the material can no longer be evaluated. For aesthetic reasons as well, there is often a requirement to use sensor surfaces that have an overprint or a mask with a cutout. For example, sensor surfaces made from metal panels, metalized foils or with vacuum metalized surfaces in which characters or symbols are cut out are used. There are also surfaces that have been painted and on which symbols are visible due to mask formation. These cutouts or masks in the impermeable surfaces allow light to exit. But they are often very small, which means that the conventional optical measuring methods, which are based on reflection, cannot be used, because the attenuation values of the sensor surfaces themselves are too high. Moreover, such operating surfaces are often not suitable generally for capacitive measuring methods or other measuring methods. It is also often intended that the buttons should be illuminated by a light source from behind, so that the shape of the cutout is illuminated on the surface.